SSL_CTX_set_client_cert_cb, SSL_CTX_get_client_cert_cb - handle client
certificate callback function
#include <openssl/ssl.h>
void SSL_CTX_set_client_cert_cb(SSL_CTX *ctx,
int (*client_cert_cb)(SSL *ssl, X509 **x509,
EVP_PKEY **pkey));
int (*SSL_CTX_get_client_cert_cb(SSL_CTX *ctx))(SSL *ssl, X509 **x509,
EVP_PKEY **pkey);
SSL_CTX_set_client_cert_cb() sets the
client_cert_cb callback,
that is called when a client certificate is requested by a server and no
certificate was yet set for the SSL object.
When
client_cert_cb is NULL, no callback function is used.
SSL_CTX_get_client_cert_cb() returns a pointer to the currently set
callback function.
client_cert_cb is the application defined callback. If it wants to set a
certificate, a certificate/private key combination must be set using the
x509 and
pkey arguments and "1" must be returned. The
certificate will be installed into
ssl, see the NOTES and BUGS
sections. If no certificate should be set, "0" has to be returned
and no certificate will be sent. A negative return value will suspend the
handshake and the handshake function will return immediately.
SSL_get_error(3) will return SSL_ERROR_WANT_X509_LOOKUP to indicate,
that the handshake was suspended. The next call to the handshake function will
again lead to the call of
client_cert_cb. It is the job of the
client_cert_cb to store information about the state of the last call,
if required to continue.
During a handshake (or renegotiation) a server may request a certificate from
the client. A client certificate must only be sent, when the server did send
the request.
When a certificate was set using the
SSL_CTX_use_certificate(3) family of
functions, it will be sent to the server. The TLS standard requires that only
a certificate is sent, if it matches the list of acceptable CAs sent by the
server. This constraint is violated by the default behavior of the OpenSSL
library. Using the callback function it is possible to implement a proper
selection routine or to allow a user interaction to choose the certificate to
be sent.
If a callback function is defined and no certificate was yet defined for the SSL
object, the callback function will be called. If the callback function returns
a certificate, the OpenSSL library will try to load the private key and
certificate data into the SSL object using the
SSL_use_certificate()
and
SSL_use_private_key() functions. Thus it will permanently install
the certificate and key for this SSL object. It will not be reset by calling
SSL_clear(3). If the callback returns no certificate, the OpenSSL
library will not send a certificate.
SSL_CTX_get_client_cert_cb() returns function pointer of
client_cert_cb or NULL if the callback is not set.
The
client_cert_cb cannot return a complete certificate chain, it can
only return one client certificate. If the chain only has a length of 2, the
root CA certificate may be omitted according to the TLS standard and thus a
standard conforming answer can be sent to the server. For a longer chain, the
client must send the complete chain (with the option to leave out the root CA
certificate). This can only be accomplished by either adding the intermediate
CA certificates into the trusted certificate store for the SSL_CTX object
(resulting in having to add CA certificates that otherwise maybe would not be
trusted), or by adding the chain certificates using the
SSL_CTX_add_extra_chain_cert(3) function, which is only available for
the SSL_CTX object as a whole and that therefore probably can only apply for
one client certificate, making the concept of the callback function (to allow
the choice from several certificates) questionable.
Once the SSL object has been used in conjunction with the callback function, the
certificate will be set for the SSL object and will not be cleared even when
SSL_clear(3) is being called. It is therefore mandatory to destroy the
SSL object using
SSL_free(3) and create a new one to return to the
previous state.
ssl(7),
SSL_CTX_use_certificate(3),
SSL_CTX_add_extra_chain_cert(3),
SSL_get_client_CA_list(3),
SSL_clear(3),
SSL_free(3)
Copyright 2002-2020 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (the "License"). You may not use
this file except in compliance with the License. You can obtain a copy in the
file LICENSE in the source distribution or at
<
https://www.openssl.org/source/license.html>.